CHAPTER XV. 



ARACHNOIDEA AND PAL^OSTRACA. 



Class ARACHNOIDEA Spiders, Scorpions, Mites, &c. 



The class Arachnoidea is far from being a coherent unity. 

 Its subdivisions are numerous and diverse, and a statement 

 of general characters is consequently difficult. 



The anterior segments, about seven in number ', are fused into 

 a cephalothorax, which bears six pairs of appendages. The 

 most anterior of these appendages may be turned in front of the 

 mouth, but there are no pre-oral outgrowths like the antennce 

 of Insects and Myriopods. The first two pairs of appendages 

 (chelicer<z and pedipalps) generally have to do with seizing and 

 holding the food ; the others are walking legs. But although 

 six pairs occur in most, there may be more or less. The 

 abdomen is generally, but not always, without appendages ; it 

 may be segmented or unsegmented; it is generally distinct from, 

 but may be fused to, the cephalothorax. A plate-like internal 

 skeleton, called the endosternite, is often present. Respiration 

 may be by tubular trachece, or by lung books (chambered 

 trachea ?), or by both, and many would include the Branchiate 

 Paheostraca along with Arachnoidea. In the tracheate forms 

 there are never more than four pairs of stigmata. An elon- 

 gated dorsal heart usually lies in the abdomen. The position 

 of the genital aperture or apertures is usually on one of the 

 anterior abdominal segments. Except Tardigrada, all have 

 separate sexes. 



Order i. SCORPIONID^E 



Scorpions are elongated Arachnids, restricted to warm 

 countries, lurking under stones or in holes during the day, 



